When normal breathing is impaired for a patient, either because of pathological problems with the patient's lungs, such as high airway resistance or lung stiffness, or because of other extra-pulmonary physiological problems, such as paralysis due to poliomyelitis, head injuries, and the like which prevent the patient from maintaining proper respiration, a mechanical system that aids the patient in breathing is generally required. A ventilator that supplies and/or withdraws respiration gas to and from the patient is such a system. To be truly effective in supplying gas, a ventilator generally needs a prescription to adapt the ventilation to suit the particular patient. This ventilator prescription is supplied by a clinician.
A ventilator prescription by a clinician involves selecting, inter alia, an initial tidal volume, flow rate (and hence inhale time), inhale pause time, and exhale time. After the initial selection, fine adjustments, largely based on subsequent blood gas measurements, are made to these initial selections. Breathing rate and minute ventilation may be computed from these parameters. If the adjustments are properly made, some patient interaction with the ventilator is possible. For example, the proper setting of triggering sensitivity and upper pressure limit allows the patient some control over the ventilator's operation. Such patient accommodation, though difficult to achieve in a ventilator, especially those in the prior art, is highly desirable.
In prior ventilation art, inhalation delivery appears to be controlled by a time-dependent function only. These conventional ventilators generate either flow, and hence volume, or pressure as a constant or time-dependent waveform during inhalation. Therefore these ventilators can be categorized as a volume ventilator or a pressure ventilator.
The pressure ventilator is characterized by a system that terminates inhalation when a preset pressure limit is reached. In contrast, the volume ventilator is characterized by a system that terminates inhalation when a preset volume is reached, volume being flow in a finite time period. There are also time-cycled ventilators, which in general are functionally equivalent to one of the above two types.
Common to these prior art ventilators are the mechanisms for transfer of air. The positive transference of air, or delivery of air, is generally provided by one of two types of mechanisms: a pressurized reservoir or a positive displacement pump. Depending upon the method of implementation, either of these may deliver air at a predetermined flow rate or at a flow rate determined by the pressure at the airway opening.
Another feature common to most ventilators is a mechanism in the cycling logic known as triggering. At the end of exhalation, air pressure in the tubing which is below a normal minimum value is detected. This detection in turn causes an early initiation of the next inspiratory cycle. This triggering feature is adjustable to make it less susceptible to pressure artifacts.
A prime concern of any ventilator is accommodation to patient effort. Accommodation to patient effort, or control by the patient, is defined primarily as synchronization of the ventilator's inhale and exhale phases with the phases of the patient's efforts. Secondarily it means the ability to deliver air at the rate desired by the patient. In other words, the ventilator with patient effort accommodation allows the patient some control over the ventilator's operation. Such patient effort accommodation in the ventilators in the prior art, however, is achieved, if at all, only with great difficulty. In contrast, the present invention achieves an accommodation to patient effort not only superior to but also easier than that in prior art ventilators. The ventilator in accordance with the present invention accomplishes this by employing a novel pressure-volume control law. One beneficial result of accommodating patient effort is to mitigate the frequently traumatic process of initiating mechanical ventilation. For example, through augmentation of the patient's efforts and gradually increasing the tidal volume and slowing the rate to that preset on the ventilator, the patient can be made to relax and allow his ventilatory pattern to be synchronous with that of the ventilator.
If a ventilator control law completely ignores the patient's attempts to control breathing, as in the case with prior art ventilators not having patient effort accommodation, ventilation fighting occurs. This fighting, unfortunately, is often expediently resolved by simply reducing the patient's drive to breathe, such as by heavy sedation. This expediency leads to the necessity of intense monitoring of the ventilated patient to ensure that the controlled ventilation remains appropriate. Both the fighting and the means to control it in the prior art are usually unsatisfactory and have undesirable side effects that impair the patient's recovery. The ventilator using a pressure-volume control law in accordance with the invention overcomes these undesirable side effects.
In addition to the lack of satisfactorily controlling fighting, the prior art ventilators have other attributes that also contribute to unsatisfactory performance. One great contributor is the transducers used in measuring the flow of the respiration gas to and from the patient. It is necessary to monitor flow to determine airway resistance and lung compliance and to servo control the flow of air to or from the patient. Ventilators in the prior art often use flow transducers to do this monitoring of flow. For accurate measurements in such systems, the flow transducer is positioned close to the patient's airway; however, water vapor or sputum in the patient's airway tends to clog or otherwise compromise the accuracy of the flow measurements. Flow transducers thus are susceptible to error. This is especially a problem on the negative transference, or exhale, side of the circuit.
In the prior art ventilators, exhalation regulation has also been unsatisfactory. Other than providing an optional variable end-expiratory pressure or mechanical retardation, these ventilators do not really regulate exhalation. A two-phase pressure control method for regulating exhalation has been disclosed in the prior art in U.S. Pat. No. 3,961,627, entitled "Automatic Regulation of Respirators" and issued to Ernst et al. However, with this method pressure is controlled as a function of time, so that when flow rate varies, the optimal back pressure is not attained. For example, optimal back pressures for certain diseases, such as emphysema, are dependent on lung volume. Therefore, in such cases, regulation with time as the independent variable is highly undesirable.
Still another area of unsatisfactory performance in the prior art ventilators is in the handling of a cough. When a patient coughs, his respiratory requirements change immediately. Since a cough is a complex, highly coordinated act that results in the rapid expulsion of alveolar gas at a very high velocity, the cough requires a spontaneous reaction in the ventilator to account for it. Although the cough is useful in sweeping the airway free of irritant gases, dust, smoke, excess mucus, cell debris or pus, it can be problematic. For example, the patient develops high intrathoracic and intrapulmonic pressures with a forced expiratory effort against a closed glottis during a cough. The glottis then opens abruptly so that there is a large difference between alveolar pressure and upper tracheal pressure. This results in a very large flow rate for the cough. The compression of the intrathoracic airways due to high intrathoracic pressures also increases the gas velocity. However, a prior art ventilator typically interferes with this coughing process. Because tubing prevents the constriction of the glottis during a cough, an intubated patient on a prior art ventilator will not be able to cough effectively. Manual suctioning of the airway using a catheter must then be performed on the patient. However, suctioning is generally only effective in a limited distribution of the airways; it is also traumatic to the airway mucosa. Atelectasis and severe acute circulatory disturbances may occur during suctioning, because the catheter connects the interior of the trachea to the wall suctioning. Furthermore, the catheter can also occlude the endotracheal tube through which it passes. Profound hypoxia, vagal stimulation, acute right-heart strain and left-heart loading can accompany the very low lung volumes and intrathoracic pressures which will result from suctioning. All this can be avoided with the preferred embodiment that can provide an artificial cough.
Another disadvantage in the prior art ventilators is the difficulty in weaning the patient from a ventilator. In the normal course of events, all patients eventually must make the transition from ventilator support to spontaneous breathing. During this transition or weaning period, conflicts usually arise between the patient's and the ventilator's control systems. The patient may be making transient or irregular efforts such as sighing or coughing, or he may be making a sustained effort to increase the tidal volume and/or breathing rate. For most patients, these efforts to resume control of respiration are considered desirable by the clinician. Conflicts occur because, for most patients, the ability to resume regulation of breathing precedes the resumption of the ability to do the work of breathing. Respiratory muscle augmentation is still needed, but the prior art ventilator's inflexible, time-dependent control system cannot permit the patient to control the pattern of breathing.
In prior ventilation art, variable assistance provided by the ventilator is controlled using intermittent mandatory ventilation (IMV). A gas source is provided to the patient for spontaneous breathing, and the ventilator provides controlled breaths intermittently. The rate of controlled breaths and the tidal volume is variable, usually as a selectable setting. In some systems the mandatory breaths can be patient-initiated. This is termed synchronous IMV, or SIMV. The patient-initiated breaths are not necessarily more physiological in nature or more comfortable to the patient, because it requires developing significant negative pressures without reversed air flow to initiate them. Furthermore, intermittent mandatory ventilation in the prior art generally has the disadvantage of unexpectedly delivering an occasional forced assistance.